Solid propellant employing a polymer containing a carboranyl group

ABSTRACT

This application discloses a solid rocket propellant composition containing as its principal ingredients a major amount of finely divided oxidizer and a minor amount of a fuel binder. The fuel binder is essentially a cured condensation polymer of a dicarboxylic acid and a diol containing a carboranyl group. To facilitate curing the polymer may be provided with isocyanate terminals. Additives such as metal powders, e.g., aluminum powder; plasticizers, e.g., isopropyl carborane; burning rate modifiers, catalysts, etc., may be incorporated in the composition.

United States Patent H 1 3,627,596

[ 72] Inventor Joseph Green [56] References Cited [21] A I N gz g UNITED STATES PATENTS [22] f 12 1967 3,306,933 2/1967 Grafstein et al. 149/22 x [45] Patented Dec. 14, 1971 Primary ExaminerBenjamin R. Padgett [73] Assignee Thiokol Chemical Corporation A t ney-Go o K- Li r Bristol, Pa.

ABSTRACT: This application discloses a solid rocket propel- OLHD PROPELLANT EMPLOYING A POLYMER lant composition containing as its principal ingredients a CONTAINING A CARBORANYL GROUP major amount of finely divided oxidizer and a minor amount 8 Claims, No Drawings ofa fuel binder. The fuel binder is essentially a cured c0nden- 52 l sation polymer of a dicarboxylic acid and a diol containing a 1 CI i g ii carboranyl group. To facilitate curing the polymer may be [SI] C] C66! provided with isocyanate terminals. Additives such as metal [50] Fieid 149/22 19 powders g aluminum powder; plasticizers g isopropyl d carborane; burning rate modifiers, catalysts, etc.. may be incorporated in the composition.

SOLID PROPELLANT EMPLOYENG A POLYMER CONTAINING A CARBORANYL GROUP This invention relates to solid propellants adapted to be used as rocket fuels and more particularly to solid propellants comprising fuel binders containing carboranyl polymers, especially condensation polymers of dicarboxylic acids and bishydroxyalkyl carborane. The propellants of the invention are characterized by exceptionally high burning rates.

Solid propellants commonly comprise a major proportion of particulate inorganic oxidizer, a minor amount of organic fuel binder and minor amounts of various special purpose additives such as burning rate modifiers, catalysts, plasticizers, pigments and wetting agents. Conventional solid propellants usually have burning rates of the order of 0.2 to 0.4 inch per second. While such burning rates are acceptable for certain applications, there are other applications for which it would be desirable to have a propellant with a substantially higher burning rate.

Various materials have heretofore been proposed for incorporation in solid propellants to increase the burning rates thereof. However, in general, such materials have been either relatively ineffective or have produced undesirable side effects such as adverse effects on the physical properties of the propellant or increases in its impact sensitivity. For example, boron hydrides such as decaborane, because of their high heats of combustion, are capable of increasing the burning rate of a propellant in which they are incorporated. HOwever, propellants containing decaborane have a high impact sensitivity, and if they contain a high concentration of decaborane, have a tendency to detonate upon ignition.

It is accordingly an object of the present invention to provide a solid propellant having a high and stable burning rate. It is another object of the invention to provide a high burning rate solid propellant that can be safely stored and handled. it is still another object of the invention to provide a high burning rate propellant that can be safely stored and handled. It is still another object of the invention to provide a high burning rate propellant that can be safely used over a wide range of combustion chamber pressures. It is a still further object of the invention to provide a solid propellant having good mechanical properties. Other objects and advantages of the invention will be in part obvious and in part pointed out hereafter.

The objects and advantages of the invention are achieved in general by utilizing as the fuel binder of a solid propellant a material that is essentially a condensation polymer of a dicarboxylic acid and a bis-hydroxyalkyl carborane having the formula:

wherein R and R are divalent, aliphatic hydrocarbon radicals, preferably having one to four carbon atoms. The preferred carborane is bis-hydroxymethyl carborane.

The present polymers may be prepared from any of various dicarboxylic acids such as for example, malonic, oxalic, succinic, glutaric, adipic, sebacic, phthalic, isophthalic, terephthalic maleic, fumaric, dimer and itaconic acids. Carborane diols other than bis-hydroxymethyl carborane may be used such as, for example, bis-hydroxyethyl, propyl, propenyl and butyl carboranes. The hydrocarbon radicals between the hydroxyl groups and the carborane nucleus of the carborane diols may be the same or different. Illustrative methods of making the condensation polymers are given below.

In general, the propellant compositions of the invention are formulated by mixing with a major amount of inorganic oxidizer a minor amount of the condensation polymer as a fuel binder, together with a curing agent for the polymer and optionally various special purpose ingredients that are conventionally used in solid propellants. Any of the solid oxidizers known to be useful in solid rocket propellants can be used in the present compositions. However, the preferred oxidizers are ammonium and potassium perchlorates and mixtures thereof. As indicated in the specific examples set forth below, especially good results have been obtained in the present compositions by using a mixture of approximately equal quantities of ammonium an potassium perchlorates.

The polymer embodied as a fuel binder in the present compositions can be cured by cross-linking agents known to be useful in cross-linking hydroxyl and carboxyl-terminated polymers, such as polyepoxides and polyimines. However, it has been found preferable in most cases to provide the polymer, prior to incorporation in the propellant mixture, with isocyanate terminals. This can be done by reacting the condensation polymer with any of various diisocyanates such as for example, 2,4-toluene diisocyanate; an :20 mixture of 2,4- and 2,6-toluene diisocyanate; dianisidine diisocyanate; hexamethylene-l ,o-diisocyanate; 4,4'-diisocyanatodiphenylmethane; and 3,3-bitolylene-4,4'-diisocyanate. The use of an isocyanate-terminated polymer permits curing of the composition at a lower temperature and also permits a wider variety of curing agents to be used. Suitable cross-linking agents for curing the isocyanate-terminated polymer are dihydroxyethylethylenediamine; ethanolamine; hexamethylenediamine; 1,2,6-hexanetriol; 4,4'-methylene-bis( 2- chloroaniline); trimethylolpropane; tris(hydroxymethyl)- nitromethane; castor oil (glyceryl triricinoleate). Several curing systems that have been found especially useful are illustrated in the examples.

It has been further found that both the physical properties and the burning rate of the present compositions can be improved by incorporating therein a plasticizer which is isopropyl carborane. While the use of isopropyl carborane is not essential, it has been found generally advantageous. Also other known additives such as aluminum or magnesium powder, burning rate modifiers, catalysts, etc. may be incorporated in the mixture. Upon completion of the mixing operation, the mixture is cast into a desired configuration and cured in situ in known manner to form the solid propellant.

In general, the relative proportions of the several ingredients of the propellant composition used are not particularly critical and may be varied considerably, provided that the oxidizer is used in a major amount and the fuel binder in a minor amount. It is usually desirable to maintain the proportions within the following ranges in parts by weight:

Oxidizer 60 to 80 Fuel binder 10 to 25 Other ingredients if used:

Plasticizer 5 to 25 Metal powder 5 to 25 Additives such as burning rate catalysts, pigments,

wetting agents, etc. 0 to 5 In order to point out more fully the nature of the invention a number of specific examples are given below illustrating the preparation and testing of several propellant compositions embodying the present invention. The examples illustrate the use as fuel binders of several condensation polymers that were prepared as follows:

Polymer A A l2-liter polymerization kettle fitted with a stirrer, heating bath, thermometer and Dean-stark condenser was charged with a mixture of 8170 g. (40.0 moles) of bis(hydroxy methyl)carborane, 2470 g. (36.0 moles) of adipic acid and 1500 ml. of xylene. Nitrogen gas was introduced under the surface of the mixture to agitate it and the mixture was heated with stirring at a pot temperature of l60200 C. for hours. During the last 24 hours of heating, the Dean-Stark condenser was removed and a collecting condenser substituted. This allowed all of the xylene solvent to be removed from the reaction mixture. A sample of the product was titrated at this point and was determined to have an acid number of 3.3. The molecular weight of a sample was 2500. The total weight of the product was 8,700 g.

To the kettle containing 8,700 g. of polymer was added 8,700 g. of isopropenylcarborane and the materials were mixed at 80 C. until solution was effected. To the solution was added 1,310 g. (7.67 moles) of toluene diisocyanate and 3.0 g. of o-chlorobenzoyl chloride. This mixture was stirred and heated at 90 C., while under a blanket of nitrogen, for 4 hours. A sample of the product was titrated and found to contain 2.03 percent isocyanate.

Polymer B This polymer was prepared in the same general manner as polymer A except that isopropylcarborane rather than isopropenylcarborane was used as plasticizer. The quantities of reactants, yield and certain properties of the polymer are given below:

Bis(hydroxy methyHcarboranc 9,072 g. 44.4 moles Adipic acid 5,802 g. 39.7 moles Xylene 1,600 ml. Yield of polymer 12,229 g. Molecular weight of polymer 2,510 Acid number of olymer 3.4 lsopropylcarborane (plasticizer) 12,229 g. Toluene diisocyanatc 1,830 g. 10.7 moles o-Chlorobenzoyl chloride 3.4 g.

Bisfliydroxy methyl)carborane 300 g.

Adipic acid 238 g. 1.63 moles Xylene 150 ml. Yield of polymer 452 g. Acid number of polymer 35 lsopropyl carbornne 452 g.

Polymer D This polymer was prepared in the same general manner as polymer B except that no plasticizer was used and no conversion of the reactive terminals to isocyanate was effected. The quantities of reactants, yield and certain properties are given below:

Bis(hydroxy mcthyl)carborane 9,072 g. 44.4 moles Adipic acid 5,802 g. 39.7 moles Xylene 1,600 ml.

Yield of polymer 12,229 g.

Molecular weight of polymer 2.510

Acid number of polymer 3.4

Polymer E This polymer was prepared in the same general manner as polymer A except that isopropylcarborane was used in place of isopropenylcarborane as the plasticizer. The quantities of reactants, yield and certain properties are given below:

Bis(hydroxy mcthyl)carborane 7,050 g. 34.6 moles Adipic Acid 4,540 g. 31.1 moles Xylene 1,400 ml. Yield of polymer 9,700 g. Acid number of polymer 3.13 g. isopropylcarboranc 9,700 g. Toluene diisccyanate 1,385 g. 8.1 moles o-chlorobenzoyl chloride 2.8 g.

Final product contained 2.2 percent isocyanate Example 1 A conventional mixer was charged with the following materials in the quantities indicated in parts by weight: 71.6 parts of ammonium perchlorate having a particle size of 300 to 800 microns, 15.2 parts ofpolymer A, l 1 parts of aluminum powder having a particle size of about 40 microns, 1.0 parts of lecithin, 0.67 parts of castor oil, 0.1 parts of butanediol, 0.06 parts of ferric acetyl acetonate and 0.4 parts of Kosmos carbon black. The foregoing ingredients were thoroughly mixed and the resulting mixture was cast and cured at a temperature of F. for a period of 16 hours. The mixture cured to form a rubbery matrix in which the particles of oxidizer and aluminum powder were uniformly distributed. In this composi tion the lecithin is used as a wetting agent to improve the mixing of the ingredients. The butane diol, castor oil and ferric acetyl acetonate comprise the curing system.

The cured propellant was tested to determine its burning rate at 500 p.s.i. and 1,000 p.s.i. pressures. Also the exponent N of the burning rate equation for the propellant was determined. The value of the pressure exponent n ofa solid propellant is indicative of the stability of the propellant. Solid propellants having pressure exponents that approach unity tend to have unstable burning characteristics.

The solid propellant of this example was found to have a burning rate of 1.33 inch/sec. at 500 p.s.i. and 1.84 inch/sec. at 1,000 p.s.i. The pressure exponent n was found to be 0.45 in the 450-600 p.s.i. pressure range thus indicating that this propellant had a relatively low tendency to detonate during burning. Certain other properties of the propellant such as tensile strength, storability and resistance to cracking when subjected to temperature cycling were determined and found to be comparable to those of a conventional propellant. The measured burning rates are of the order of three to 10 times as great as those of a conventional prior art propellant.

Example 2 A mixer was charged with the following ingredients in parts by weight: 18.2 parts of polymer B, 69 parts of potassium perchlorate having a particle size of the order of 300 to 800 microns, l 1 parts of powdered aluminum of about 40 micron size, 1.0 parts oflecithin, and a curing system comprising 0.06 parts of 1,4-butane diol, 041 parts of castor oil and 0.26 parts of dibutyl tin dilaurate. These ingredients were mixed, cast and cured as in example 1 and tested to determine the burning rate and pressure exponent of the cured composition. The burning rates of this composition were found to be 2.54 inch/sec. at 500 p.s.i. and 4.10 inch/sec. at 1,000 p.s.i. The pressure exponent was 0.69 in the 200-1,600 p.s.i. pressure range.

Example 3 A mixer was charged with the following ingredients in the indicated parts by weight: 34.5 parts of ammonium perchlorate, 34.5 parts of potassium perchlorate, 9.105 parts of polymer B, 9.105 parts of isopropyl carborane, l 1 parts of aluminum powder, 0.06 parts of 1,4-butane diol, 0.26 parts of dibutyl tin dilaurate, 0.41 parts of castor oil and 2.0 parts of ammonium dichromate. The foregoing ingredients were thoroughly mixed and the resulting mixture, cast, cured and tested as in example 1. In this composition the ammonium dichromate is a burning rate catalyst.

It was found that the use of a mixture of ammonium and potassium perchlorates provided a solid propellant with the exceptionally high burning rates of 3.8 inch/sec. at 500 p.s.i. and 6.8 inch/sec. at 1,000 p.s.i. The use of isopropyl carborane as a plasticizer improved the physical properties of the propellant and also contributed to the high-burning rates achieved. The pressure exponent of this propellant was found to be 0.84 in the 200-1 ,300 p.s.i. pressure range, which value, although higher than the pressure exponents of the propellants of examples 1 and 2, is still an acceptable value. The physical properties of the propellant were comparable to those of a conventional propellant.

Example 4 A solid propellant was prepared by mixing the following ingredients and casting and curing the resulting mixture: 62 parts of ammonium perchlorate, five parts of potassium perchlorate, 12.5 parts of polymer C, 12.5 parts of isopropyl carborane, eight parts of aluminum and two parts of ammonium dichromate.

The burning rates of this solid propellant were 1.9 inch/sec. at 500 p.s.i. and 3.25 inch/sec. at 1,000 p.s.i. and its pressure exponent was 0.77 in the 200-2,000 p.s.i pressure range.

Again, good physical properties of the solid propellant were noted. Example 5 A solid propellant was prepared from the following ingredients: 69 parts of ammonium perchlorate, l0 parts of polymer D, parts of isopropyl carborane, ll parts of aluminum powder and two parts of ammonium dichromate. The burning rates of this propellant were found to be 1.57 inch/sec. at 500 p.s.i. and 2.47 inch/sec. at 1,000 p.s.i. The pressure exponent of this propellant was 0.70 in the 2001 ,300 p.s.i. pressure range. The physical properties of the propellant were similar to those of the propellants of the preceding examples.

Example 6 A solid propellant was prepared as in accordance with the procedure of example I from the following ingredients: 14.72 parts of polymer E, 70 parts of ammonium perchlorate, 0.25 part of lecithin, parts of aluminum powder, 0.26 part of hexane triol and 0.025 part of ferric acetyl acetonate. The cured solid propellant exhibited burning rates of 2.08 inch/sec. at 500 p.s.i. and 3.38 inch/sec. at 1,000 p.s.i. The pressure exponent of the propellant was found to be 0.7 in the 250-3,000 p.s.i. pressure range. The physical properties of this propellant were similar to those of the preceding examples.

From the foregoing examples it should be apparent that solid propellants embodying the present invention have exceptionally high burning rates, in some cases as much as 10 to 30 times as great as those of conventional propellants. Moreover, they have relatively low-pressure exponents, and physical properties that permit them to be used in rocket motors having a variety of grain designs and a variety of operating conditions.

it is of course to be understood that the foregoing examples are intended to be illustrative, and that numerous changes can be made in the ingredients, proportions and conditions set forth therein without departing from the spirit of the invention as defined in the appended claims.

I claim:

1. A solid propellant comprising a major amount of finely divided inorganic oxidizer and a minor amount of a fuel binder consisting essentially of a condensation polymer of a dicarboxylic acid and a bis-hydroxyalkyl carborane.

2. A solid propellant according to claim 1 containing a minor amount of a plasticizer which is isopropyl carborane.

3. A solid propellant according to claim 1 containing a minor amount of aluminum powder.

4. A solid propellant according to claim 1 and wherein said dicarboxylic acid is adipic acid.

5. A solid propellant according to claim 1 and wherein said carborane is bis-hydroxymethyl carborane.

6. A solid propellant according to claim 1 and wherein said oxidizer is selected from ammonium perchlorate, potassium perchlorate and mixtures thereof.

7. A solid propellant according to claim 6 and wherein said oxidizer comprises approximately equal amounts of ammonium perchlorate and potassium perchlorate.

8. A solid propellant comprising from 60 to parts by weight of finely divided inorganic oxidizer, 10 to 25 parts by weight of a fuel binder consisting essentially of a condensation polymer of a dicarboxylic acid and a bis-hydroxylalkyl carborane, five to 25 parts by weight of isopropyl carborane and five to 25 parts by weight of aluminum powder. 

2. A solid propellant according to claim 1 containing a minor amount of a plasticizer which is isopropyl carborane.
 3. A solid propellant according to claim 1 conTaining a minor amount of aluminum powder.
 4. A solid propellant according to claim 1 and wherein said dicarboxylic acid is adipic acid.
 5. A solid propellant according to claim 1 and wherein said carborane is bis-hydroxymethyl carborane.
 6. A solid propellant according to claim 1 and wherein said oxidizer is selected from ammonium perchlorate, potassium perchlorate and mixtures thereof.
 7. A solid propellant according to claim 6 and wherein said oxidizer comprises approximately equal amounts of ammonium perchlorate and potassium perchlorate.
 8. A solid propellant comprising from 60 to 80 parts by weight of finely divided inorganic oxidizer, 10 to 25 parts by weight of a fuel binder consisting essentially of a condensation polymer of a dicarboxylic acid and a bis-hydroxylalkyl carborane, five to 25 parts by weight of isopropyl carborane and five to 25 parts by weight of aluminum powder. 